A disagreement over a well-known slice of the Southern California coast is threatening to drive a wedge between Marines and surfers, groups that had recently set aside differences and become political allies.

At issue is the 2.25-mile stretch of surf and sand known as Trestles, between the San Onofre nuclear plant and the San Diego County-Orange County line. The name comes from two train trestles that parallel the ocean.

To wave riders, Trestles represents seven of the primo surf breaks in the world. To Marines, the middle section of the 2.25 miles is an ideal location to teach grunts how to fight their away from ship to shore and inland.

With San Clemente's Surfrider Foundation in the lead, surfers petitioned to have Trestles listed on the National Register of Historic Places, for its role in the rise of "surf culture." Surfers hope the listing will ensure that nothing will disrupt the site's isolation and lack of "commercial growth."

The Marine Corps, which owns the beach and the paths leading to it, opposes having Trestles listed, out of concern that the designation might lead to civilian oversight that crimps training.

The skirmish is not over, but so far the surfers appear to be winning.

Despite opposition from the Marine Corps and the Navy, the State Historical Resources Commission voted unanimously Feb. 8 to forward a recommendation to Washington that Trestles be listed.

The decision now rests with the civilians who run the National Register, which is part of the National Park Service.

The recommendation "does not and will not impose any additional requirements for consultation for military training and operational use," according to documents supporting the Trestles nomination.

"This is a historic designation, it does not change the use of the property," said Amy Crain, a state historian.

But military brass and two state senators aren't buying the assurances that nothing will keep the Marines from using what is called Green Beach for mock assaults, or for other exercises involving heavy vehicles traveling between the beach and the open spaces of Camp Pendleton.

Military officials "believe these assurances to be unenforceable," Brig. Gen. Vincent Coglianese, commanding general of Camp Pendleton, wrote to the state historic preservation officer before the commission vote.

State Sens. Mimi Walters and Mark Wyland, in a joint statement, warned that listing Trestles will "put state bureaucrats and surfers in control of Marine Corps training near Trestles."

From World War II to 1971, the Marine Corps was in control of the entire 2.25 miles. But after prodding from then-President Nixon, a part-time resident of San Clemente, the military leased the property to the state to establish San Onofre State Beach.

Of course, even before the Trestles breaks - Uppers, Lowers, Middles, Church, the Point, Old Man's and Dog Patch - were open to the public, intrepid surfers were attracted by the near-perfect waves.